High energy metal fuel and process for producing same



United States Patent N 0 Drawing.

This invention relates to powdered, high energy, metal fuels, such as are presently being used as components of solid fuel mixtures for powering reaction motors. It also relates to processes for producing these fuels.

Various high energy metals are being powdered for use as rocket fuels. While it is highly desirable that the powder particles be reduced to low micron or sub-micron size, various problems are encountered in doing so. Ordinary comminution procedures are inadequate. Other known procedures are too time consuming and expensive for large scale production. But perhaps the greatest difiiculty is the pyrophoric nature of the product, particularly when the particles are thin leafed or flaked in character'in order to expose the maximum surface area for a given weight.

This invention is concerned not only with producing, on a large scale basis, sub-micron particles of a high energy metal, or alloy made up principally of a high energy metal, but with preventing oxidation thereof during and following production.

I have found that ordinary particles of a high energy metal, or alloy made up principally of a high energy metal can be reduced in size to within an unusually low micron and sub-micron range and can be made extremely thin in leaf or flake form as platelets, while being effectively protected from oxidation, if they are subjected to extremely high pressure impacts while immersed in certain non-reactive liquids.

Liquids which have been found suitable are certain organic solvents, certain polymers, and certain plasticizers.

A polymer presently used in the compounding of rocket fuels is polybutadiene-acrylic acid, often spoken of as PBAA. I have found that this serves as an excellent liquid medium for the purpose and renders the product suitable for immediate use as a rocket fuel component.

A plasticizer presently used in the compounding of rocker fuels is triacetin. I have found that this also serves as an excellent liquid medium and similarly renders the product suitable for immediate use as a rocket fuel component.

Organic solvents found useful as liquid media for the purpose are ethyl acetate and acetone. Since these are not ordinarily used as components of rocket fuel mixtures, they may be vaporized from the powdered metal following the comminuting operation in all instances where their presence would be undesirable.

It is ordinarily desirable to dilute the polymer or plasticizer. Acetone or ethyl acetate may be used for this purpose, as may also such organic solvents as toluene and methyl ethyl ketone even though ineffective when used alone.

Metals whose heat of combustion to the oxide is at least 4.00 kilo-calories per gram are generally considered to be high energy metals. Included by this definition are beryllium, boron, lithium, aluminum, magnesium, titanium, and silicon. All of these metals can be successfully processed in accordance with the invention, as can their alloys. An alloy of aluminum and magnesium, for example, has proven superior for comminuting purposes to either of the metals alone.

Comminution is most satisfactorily carried out in a high speed, planetary, counter-rotation, ball mill, wherein the force acting on the balls is increased by as much as ten times over that of ordinary ball mills.

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Using this type of ball mill and one or another of the liquid media specified above, I have been able to consistently reduce ordinary metal particles to platelets of low micron or sub-micron sizes.

Thus, for example, using a commercial aluminum metal powder of 325 mesh as the raw metal and PBAA as the liquid medium, I was able to reduce the metal particles to sizes ranging from sub-micron up to no greater than 5 microns and to end up with an intimate dispersion of these particles in the PBAA as a carrier. This product was sold to a leading producer of solid fuel for rockets as a high energy fuel component. Microscopic examination showed the particles to be thin platelets.

Again, using a similar aluminum powder as the raw metal and triacetin as the liquid medium, I was able to produce a corresponding product in which the metal was in the form of thin platelets ranging in size from submicron to 5 microns and intimately dispersed in the triacetin as a carrier. This product was sold to another leading producer of solid fuel for rockets.

Corresponding dispersions of aluminum-boron alloy, aluminum-boron-titanium alloy, and .magnesium-aluminu'm alloy platelets of from sub-micron to 5 micron size range, in both PBAA and triacetin as carriers, have been produced to meet the demand of leading producers of solid fuels for rockets.

In order to satisfy a demand for an exceedingly fine titanium metal powder, undispersed, I utilized ethyl acetate as a liquid medium and produced platelets in the 3-5 micron range. The ethyl acetate was removed by volatilization following the comminuting operation.

A similar titanium powder was produced in like manner using acetone as the liquid carrier.

Many other instances of production for both laboratory and commercial purposes have shown that particle sizes of as low as 0.01 micron can be obtained throughout the bulk of the product by carrying out the comminuting operation for a sufiiciently long period of time, say from A to ten hours, depending upon the particular material concerned and the degree of fineness desired. Elemental boron, for example, requires a considerably longer period than does aluminum.

The following constitutes a typical example of operating procedure utilizing a mill having four stainless steel pots arranged for placement about the circumference of a circular carrier adapted to rotate at 216 r.p.m. each pot being adapted to rotate on its own axis at 121 r.p.m.

Into each pot was placed 2270 gms. of inch diameter stainless steel balls and 1360 gms. of inch diameter stainless steel balls. Then, into each pot was placed a mixture of 600 gms. of commercial minus 325 mesh aluminum powder suspended in a solution of 200 gms.

ethyl acetate, 400 gms. toluene, and 50 gms. PBAA. The pots were closed, secured tightly, and placed in the mill, which was then operated continuously for a period of 4 hours. During this time, the temperature of the mixture rose to 125 F. At the end of the operating period, the pots were opened under a blanket of dry nitrogen, the grinding balls separated from the product, and the solvents removed from the aluminum in vacuo. The resulting powder was found on microscopic examination to have an average particle size of 5 microns. Each particle was intimately coated with a thin film of polymerized PBAA, which prevented oxidation.

By a similar procedure, using triacetin instead of PBAA, each particle is protectively coated with a thin waxy film of solidified triacetin.

In those instances where ethyl acetate or acetone are used, in themselves, as the liquid medium and are volatilized following the comminution procedure, I have found that the resulting dry particles are still effectively coated with a thin film protecting against oxidation.

'It should vhe understood that particle size as here presented is determined on the basis of length or breadth,

whichever is the greater.

- Whereas there is here illustrated and described certain preferred forms which I presently regard as the bestv mode of carrying out my invention, it should be understood thatvarious changes may be made without departing from the disclosed inventive subject 'matter particularly pointed out and distinctly claimed herebelow.

I claim:

1. A process for producing a high energy fuel, cm-- prising comminuting a high energy, solid material selectedfrornthe group consisting of beryllium, boron,

lithium, aluminum,'magnesiurn, titanium, silicon, and

their alloys by subjecting massed particles of said solid vmaterial to a milling operation in a high speed, planetary,

counter-rotation, ball mill which applies to said par ticles a multiplicity of rapidly repeated mechanical blows at a force several times greater than the force of gravity,

said comminuting being carried out in a non-reactive,

oxidation preventing, organic liquid selected from the group consisting of polybutadiene-acryiic acid, triacetin,

ethyl acetate, and acetone.

2. The process of claim 1, wherein the cornrninut'in'g is carried out in a non-reactive, oxidation preventing, organic'liquid selected from the group consisting of polybutadiene-acrylic acid and triacetin, said liquid being diluted by a non-reactive organic solvent selected from the group consisting ofethyl'acetate, acetone,-toluene,;

and methyl ethyl ketone.

3.-A high energy fuel, consisting essentially of particles of a high energy solid material selected from the groupconsisting of beryllium, boron, lithium, aluminum,

magnesium, titanium, silicon, "and their'alloys, said particles being mechanically worked platelets of size ranging from about 0.01 'micron to about 5 microns, each of said particles being intimately coated with a thin film of an oxidation-preventing material selected from the group consisting ofpolyrnerized polybutadiene acrylic acid and of solidified triacetin; I

References Cited by the Examiner UNITED STATES PATENTS OTHERfiREFERENCES= Anderton: Aviation Week, Nov. 12, 1,956, pages 5l-57.

- Frazier: Proceedings 14th Annual Meeting, Metal Powder Association, April 21-23 (1958'), pages -70.;

, Report, 43rd National Advisory Committee for Aeronautics, 1957, pages 24.-

Chem. and Eng. News, Jan. 19, 1959, :page 53. V

CARL D. QUARFORTH; Primary Examiner.

LEON D. ROSDOL, Examiner. 

3. A HIGH ENERGY FUEL, CONSISTING ESSENTIALLY OF PARTICLES OF A HIGH ENERGY SOLID MATERIAL SELECTED FROM THE GROUP CONSISTING OF BERYLLIUM, BORON, LITHIUM, ALUMINUM, MAGNESIUM, TITANIUM, SILICON, AND THEIR ALLOYS, SAID PARTICLES BEING MECHANICALLY WORKED PLATELETS OF SIZE RANGING FROM ABOUT 0.01 MICRON TO ABOUT 5 MICRONS, EACH OF SAID PARTICLES BEING INTIMATELY COATED WITH A THIN FILM OF AN OXIDATION-PREVENTING MATERIAL SELECTED FROM THE GROUP CONSISTING OF POLYMERIZED POLYBUTADIENE ACRYLIC ACID AND OF SOLIDIFIED TRIACETIN. 